Electroslag welding is a non-arc welding process utilizing an electrical current that passes between a consumable electrode and a work piece. When this method of welding is being performed, a liquefied slag that covers the surface of the weld conducts the electrical current. Before electroslag welding begins, a welding flux is used to fill the gap between the work pieces, and an electrical arc is used to generate the heat needed to melt the flux and form the slag. During this type of welding, the slag remains in a molten state from the heat from the electrical current.

Electroslag welding is most often used to join thick steel plates together. This method of welding is typically performed by placing two water-cooled copper retaining devices on each side of the work pieces to form a void for the molten flux. Filler wire, used as the electrode, is added to the void along with a small quantity of welding flux. An electrical arc is generated to begin the melting process, and additional quantities of flux are added until the molten slag fills the void and douses the arc. The filler wire melts into the liquefied slag and forms the weld as it solidifies; the process continues with the retaining devices and filler wire moving upwards to the end of the weld.

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A variation of the typical electroslag welding process is sometimes used in order to shorten the work time. In this variation, the welding process is conducted in the typical fashion, except that the filler wire is fed into the molten pool by means of a consumable tube. This tube is positioned at the top of the weld and feeds the wire into the molten flux in an oscillating manner for wider joints. In this variation, two sets of water-cooled copper retaining devices are utilized so that they can be moved along by leaping over each other. This variation is particularly useful when the welding process is carried out in a vertical position.

Robert K Hopkins patented the process of electroslag welding in the U.S. in 1940. This welding method was further refined at the Paton Institute in the U.S.S.R. throughout the 1940s. The refined Paton method of electroslag welding was first introduced at the Bruxelles Trade Fair in 1950 and began to be used by U.S. automaker General Motors to fabricate engine blocks in 1958. The Bank of America skyscraper in San Francisco was constructed using the electroslag welding method.

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